Which external renderer should I use for architectural visualization?
You would choose a raytracer that has features that are required for architecture projects. 

An example is YafaRay. YafaRay supports features like Global Illumination and Background Illumination which are specifically useful for architecture visualization.

Cycles has Global Illumination (since a few years ago) and I am not sure that it has as good Background Illumination as YafaRay. 

YafaRay is distributed with an open license and you can use for free. It is designed to be used with Blender.